1. Field of the Invention
The present invention relates generally to a power steering apparatus adapted to supply to an actuator of a steering mechanism pressurized liquid in an amount proportional to the degree of steering operation by utilization of a metering function of a gerotor, and particularly to a power steering apparatus in which a valve mechanism for the switchover of fluid paths which communicate with a gerotor is improved in processibility to contribute to reduction in cost.
2. Description of the Prior Art
A conventional power steering apparatus of the type utilizing a metering mechanism of a gerotor is disclosed in U.S. Pat. No. Re. 25,126, for example. The conventional power steering apparatus is characterized by connecting a steering rotary shaft to a rotary spool freely rotatably attached to the center of a housing, installing a sleeve having the rotary spool inserted therein and at the same time connecting the sleeve to a gerotor so as to be rotated by a rotor of the gerotor, applying liquid pressure to the gerotor via the sleeve by the rotation of the spool during a steering operation, thereby rotating the rotor in synchronism with the steering operation, transmitting the rotation of the rotor to the sleeve to thereby allow the sleeve to be rotated in a follow-up fashion relative to the rotation of the spool, and supplying pressurized liquid in an amount proportional to the degree of the steering operation from a portion of the gerotor contracted by the rotation of the rotor to an actuator of a steering mechanism, thereby actuating the steering mechanism with a small operating force.
With the conventional power steering apparatus, however, since there is adopted a rotary valve mechanism wherein the sleeve rotated in a follow-up fashion by the gerotor is provided around the outer periphery of the spool rotated by the steering operation so that the spool is inserted into the sleeve, a plurality of opening grooves are required to be formed in the outer circumference of the spool in the axial direction for the purpose of allowing switchover holes bored both in the spool and in the sleeve to register with each other, and the opening portions of the switchover holes are required to be widened as spot facing holes for the purpose of sufficiently securing the area of the fluid paths. The formation of the opening grooves and the opening portions of the switchover holes requires not only perforation processing but also special processing of spot facing for forming the grooves in the axial direction. Thus, a number of processing steps are required for the spool and the sleeve which constitute the rotary valve mechanism, thereby bringing about a rise in cost. What is worse, since a plurality of opening grooves are formed between the spool and the sleeve, there gives rise to a phenomenon of leakage of the liquid from gaps between the sleeve and the spool. In order to attempt to prevent this liquid leaking phenomenon from occurring, the rise in cost is further brought about.
Further, the conventional power steering apparatus is provided with a centering mechanism for maintaining the spool and the sleeve in a neutral state which intercepts the supply of pressurized liquid to the gerotor to bring the apparatus to a standstill. The centering mechanism has a construction such that a pair of leaf springs are fitted in notched portions which are formed in a direction perpendicular to the shaft and disposed at a prescribed position of a combination of the spool and the sleeve fitted to each other, and that stopper pins are provided at different positions, fixed to the outer sleeve and loosely extending through the inner spool. Therefore, the axial lengths of the spool and the sleeve are made larger by an extent corresponding to the fact that the leaf springs and the stopper pins are provided separately. Further, it is required to process the spool and the sleeve to form therein holes for fitting the leaf springs and the stopper pins in addition to a plurality of holes and opening grooves for forming the switchover fluid paths. Thus, the conventional power steering apparatus entails a disadvantage that the valve becomes large in size and complicated in construction.